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"The steamer toiled along slowly on the edge of a black and incomprehensible frenzy. The prehistoric man was cursing us, praying to us, welcoming us—who could tell? We were cut off from the comprehension of our surroundings; we glided past like phantoms, wondering and secretly appalled, as sane men would be before an enthusiastic outbreak in a madhouse. We could not understand because we were too far and could not remember because we were travelling in the night of first ages, of those ages that are gone, leaving hardly a sign—and no memories." (2.7)
Catching sight of wild native Africans in their homeland rouses fear in the pilgrims. They feel as if they have traveled to a place where nothing is comprehensible. They cannot read the attitude of the Africans towards them. Marlow compares their mental state to that of inmates in an insane asylum right before an outbreak—teetering on the edge of insanity.
"The current ran smooth and swift, but a dumb immobility sat on the banks. The living trees, lashed together by the creepers and every living bush of the undergrowth, might have been changed into stone, even to the slenderest twig, to the lightest leaf. It was not sleep — it seemed unnatural, like a state of trance. Not the faintest sound of any kind could be heard." (2.13)
In this scene of madness, every aspect of the wilderness seems struck dumb, as if all of nature has turned to stone.
[During the fog]: What we could see was just the steamer we were on, her outlines blurred as though she had been on the point of dissolving, and a misty strip of water, perhaps two feet broad, around her—and that was all. The rest of the world was nowhere, as far as our eyes and ears were concerned. Just nowhere. Gone, disappeared; swept off without leaving a whisper or a shadow behind. (2.13)
If you've ever been in a thick fog, you might know this feeling: that you're the only one left on earth. (Stephen King wrote a pretty freaky book about it.) When you think about it, this is probably how Kurtz feels: that he's the only (white) man left on earth, or at least the only one worth caring about.